Pressure relief valve

ABSTRACT

A pressure relief valve includes a fluid flow channel (6, 19, 21) through which an initial flow of fluid passes in use, and a valve opening member (23) responsive at a predetermined fluid pressure to open the valve (13) and allow an increased fluid flow therethrough. The valve opening member (23) is located substantially centrally in the fluid flow channel (6, 19, 21) to ensure it is always exposed to the fluid flow. Part of the flow channel (21) is formed by part of the valve (13). The valve opening member (23) and flow channel (21) are elongated and parallel. A screen (13) on the valve screens the outlet end of the channel (21) from the fluid inlet port (6).

This invention relates to a pressure relief valve, particularly, but notexclusively, for an aircraft undercarriage damping arrangement.

An aircraft undercarriage arrangement is shown in FIG. 1 and comprisesan upper leg 1 mounted at a suitable point in a wing of the aircraft,and a lower leg 2 slidably located about a lower end of the upper leg 1.The lower leg 2 is connected to a wheeled bogey by a trunnion 3. Upperleg 1 is hollow to define an upper cavity 4 whilst lower leg 5 defines alower cavity is 5. The cavities are separated by an end wall 6 and thelower cavity is filled with a damping oil and the upper cavity filledwith damping oil and nitrogen gas. The oil is allowed to flow through avalve 7 mounted in the end wall 6 to permit a controlled collapsing ofthe upper leg 1 into the lower leg 2, thereby absorbing the impact oflanding.

In the event of a heavy landing, it is necessary for the valve 7 to openfully to "soften" the undercarriage and avoid the impact beingtransmitted to passengers. In the past this has been achieved byproviding a valve 7 as shown in FIG. 2, with a spring-loaded orificeplate 8 which permits an initial limited flow through the orifice 9therein, up to a predetermined pressure limit, at which thespring-loading is overcome and the plate lifts to allow an increasedflow through the valve seat 8a with which the plate cooperates.

A problem with such existing valves is that the differential area of theorifice plate may be small at high operating pressures and the oil flowpast it when the valve is open may prevent it from being held open in apositive manner.

According to the invention there is provided a pressure relief valvecomprising a fluid flow channel through which an initial flow of fluidpasses in use, and a valve opening member responsive at a predeterminedfluid pressure to open the valve and allow an increased fluid flowtherethrough, wherein the valve opening member is located substantiallycentrally in the fluid flow channel.

By locating the valve opening member substantially centrally in thefluid flow channel, said member is always exposed to the fluid flow,thereby making the valve more reliable in operation.

Preferably, the fluid flow channel is such as to reduce or eliminate anyturbulent flow effects within the valve that would cause the valveopening characteristics to vary. In particular, the fluid flow channelis made long enough so that its outlet is spaced away from a fluid inletport and associated valve member controlled by the valve opening member.Preferably, the region around the outlet end of the fluid flow channelis screened from the fluid inlet port and associated valve member. Forexample, the fluid flow channel may incorporate the valve member in theform of a valve seat at its inlet end which cooperates with the valveinlet so as to take all the flow through it when the valve is closed. Anouter peripheral portion of the valve member may be guided within anouter housing and a screen may be provided around the outlet end of thefluid flow channel and the outer housing.

Preferably, the valve opening member is elongated in a directionsubstantially parallel to the axis of the fluid flow channel such that avalve opening force is exerted on it, induced by the drag of the fluidflow in the channel. This is preferable since the cross-sectional areaof the valve opening member may be reduced producing a less turbulentfluid flow.

Conveniently, the valve opening member comprises a rod extending alongthe axis of the flow channel.

Preferably, the fluid flow channel is defined by part of the valveopening member. This is preferable because the drag generated by wallsdefining the fluid flow channel acts to open the valve permitting afurther reduction in the cross-sectional area of the central portion ofthe valve opening member.

Advantageously, the fluid flow channel has substantially parallel wallsto produce a substantially parallel fluid flow.

By producing a parallel fluid flow, turbulence is avoided which mightproduce a closing force on the valve acting in the opposite direction tothe general fluid flow direction.

A specific embodiment of the invention will now be described, by way ofexample only, with reference to the drawings in which:

FIGS. 1 and 2, as noted above, are respectively a schematic view and anenlarged fragmentary cross-sectional view of a pressure relief valveaccording to the prior art;

FIG. 3 shows a longitudinal section through a pressure relief valve inaccordance with the invention; and

FIG. 4 shows an end view of the valve shown in FIG. 3.

FIGS. 1 and 2 are described above.

With reference to FIGS. 3 and 4, a pressure relief valve comprises agenerally cylindrical body 1, closed at one end by an end wall 2 andhaving a radially outwardly extending mounting flange 3. Bolts (notshown) pass through the flange 3 to fix the valve to an end wall 9 of anundercarriage leg which is comparable to the end wall 6 in the prior artassembly of FIG. 1. The other end of the body 1 is closed by an end cap4 having a central upwards extension in which are formed four laterallyopening fluid exit ports 5. A fluid inlet port 6 is formed centrally inthe end wall 2. Pressure relief ports 7, 8 and 8a extend radiallythrough the body 1.

An outer peripheral portion 10 of the end cap 4 is threaded forengagement with threads 11 on the internal surface of the body 1.

Located within valve cavity 12 is an axially displaceable bell-shaped,valve slide 13. The valve slide 13 has axially extending peripherallands 14 and 15 which cooperate with the ports 7 and 8, respectively.The lower end wall 16 of the valve slide 13 makes contact with the endwall 2 of the body 1 under the action of an axially extendingcompression spring 17. The spring 17 acts between an inner face of theend wall 16 and a guide washer 20 loosely located in the end cap 4. Acentral aperture 19 is formed in the end wall 16 in alignment with, andof the same diameter as, the inlet port 6 in the body 1. A limitedannular sealing area 19a of the valve slide around the aperture 19 formsa seal with the end wall 2 of the body 1.

A cylindrical flow tube 21 is located coaxially within the valve slide13 and is connected to it via an outer collar 22 at its lower end. Theupper end of the flow tube 21 projects through the aperture in the guidewasher 20. The inner diameter of the flow tube 21 equals that of theapertures 6 and 19 so that they form a constant cross-section flowchannel. Centrally located within the flow tube 21 is a pressure sensor23 that is connected to the valve slide 13 by four radially extendingarms 24 joined to an outer ring 25 which is received in a recess in theinner surface of the end wall 16 and clamped therein by the collar 22 ofthe flow tube which is connected by bolts (not shown) to the end wall 16of the valve slide 13.

During normal landing conditions, damping oil flows from the lower leginto the upper leg of an aircraft undercarriage damping arrangement viainlet port 6 and the flow tube 21 of the pressure relief valve. The oilis guided laterally by means of the exit ports 5 in the end cap 4 toensure turbulent mixing within the body of the oil.

In the event of a heavy landing, the fluid flow will increase. Apressure is then exerted on an end face 26 of the pressure sensor 23 andthe arms 24, which together with the drag generated by the axiallyextending surfaces of the pressure sensor 23 and flow tube 21, opens thevalve by forcing the valve slide 13 upwards against the spring 17.

The lands 14 and 15 slide back opening ports 7 and 8 and a gap opensbetween the valve seat 19a and the valve slide 13. Oil is then free toflow through ports 6 and 8 and on through ports 7 into the upper leg, aswell as through the flow tube 21 and exit ports 5 into the upper leg.The resistance to flow experienced by the oil is thus lessened and theundercarriage enters its "softest" state. As the valve slide 13 liftsaway from the end wall 2, the space between the slide 13, the flow tube21 and the end cap 4 and washer 20 is reduced, and oil contained in thisspace flows outwards through the ports 8a into the upper leg.

As the landing impact is absorbed the flow rate and pressure within thedamping arrangement reduces, and the valve slide 13 seats on the valveseat 19a again under the action of spring 17.

It will be appreciated that the end cap 4 and washer 20 together serveto screen the outlet end of the flow tube 21 from the space within theslide member 13, thereby preventing possible turbulence at the outletend from having an adverse effect on the operating characteristics ofthe slide member. However, it may be in some applications that the flowtube 21 is long enough to distance the outlet end far enough away fromthe slide member that such adverse turbulence effects are of noconsequence, even without these components to form a screen.

In an alternative embodiment, the flow tube may be shortened. If thescreen formed by the end cap 4 and washer 20 is retained, adverseturbulence effects in the valve can still be avoided. However, the flowtube is preferably longer than its diameter and is typically longer by afactor of two or three or more.

We claim:
 1. A pressure relief valve comprising inlet means defining afluid inlet port; a valve member that is movable when a predeterminedpressure level is exceeded at said inlet port, between a valve closedposition in which said valve member is in engagement with the inletmeans and a valve open position in which said valve member is spacedaway from the inlet means, and that incorporates a channel portiondefining a fluid flow channel through which an initial flow of fluidpasses from the fluid inlet port when the valve member is in the valveclosed position; bias means that urges the valve member to the valveclosed position; and valve opening means comprising a flow sensitivemember connected to the valve member and located within said flowchannel to be responsive to fluid pressure at said inlet port and tofluid flow through said flow channel from said inlet port so as to urgethe valve member to the valve open position.
 2. A valve as claimed inclaim 1 wherein the flow sensitive member is elongated in a directionsubstantially parallel to the axis of the fluid flow channel such that avalve opening force is exerted on it, induced by the drag of the fluidflow in the channel.
 3. A valve as claimed in claim 2 wherein the flowsensitive member comprises a rod extending along the axis of the flowchannel.
 4. A valve as claimed in claim 1 wherein the fluid flow channelhas a substantially uniform cross-section to produce a substantiallyuniform fluid flow.
 5. An aircraft undercarriage damping systemincorporating a pressure relief valve as claimed in claim
 1. 6. A valveas claimed in claim 1 in which the channel portion is elongate and saidfluid flow channel through it extends between the fluid inlet port and afluid outlet port defined by outlet means.
 7. A valve as claimed inclaim 6 including an outer annular housing with the inlet means at oneend and the outlet means at the opposite end.
 8. A valve as claimed inclaim 7 in which the inlet means, valve member and outlet means togetherdefine an initial fluid flow path through the housing.
 9. A valve asclaimed in claim 6 in which the channel portion cooperates with theoutlet means.
 10. A valve as claimed in claim 7 in which the valvemember has an outer annular wall with a peripheral land portion whichcooperates with the annular housing to define a by-pass chamber withinthe housing between said wall and the inlet means, the inlet portcommunicating with the by-pass chamber as the valve member moves fromthe closed to the open position.
 11. A valve as claimed in claim 10 inwhich the housing defines an auxiliary fluid flow port with which theland portion cooperates to open said auxiliary fluid flow port as thevalve member moves from the closed to the open position.
 12. A valve asclaimed in claim 11 in which the auxiliary fluid flow port is an outletport from the by-pass chamber.
 13. A valve as claimed in claim 12 inwhich the housing defines an auxiliary fluid inlet port to the by-passchamber and the valve member has a land portion which cooperates withthe housing to open said auxiliary fluid inlet port as the valve membermoves from the closed to the open position.
 14. A valve as claimed inclaim 13 in which the auxiliary fluid inlet port is located nearer theinlet means than the auxiliary fluid outlet port.
 15. A valve as claimedin claim 6 in which the bias means is located between the outlet meansand the valve member.
 16. A valve as claimed in claim 15 in which thebias means comprises a spring which surrounds said channel portion.